In general terms, motorcycles typically include a main frame to which the engine is attached. One or more sub-frames or like elements can be movably attached to the main frame, each of which can be adapted to hold one of the motorcycle wheels. The moving sub-frames may permit the motorcycle front and rear wheels to move relative to the main frame. A suspension system may also be used to maintain the height of the motorcycle.
Traditionally, motorcycle suspension systems for street bikes have included one or more springs (typically coil springs) and a hydraulic damper unit applied to each of the front and rear wheel to provide the lift and dampening needed to suspend the vehicle and control movement of the wheels during compression and rebound of the wheels. Such a spring and damper unit may be fastened between a swing arm and main frame in the rear of the motorcycle to control the rear wheel assembly and associated parts. Also, a spring and damper unit may be positioned inside a telescoping fork assembly to control the front wheel. Another form of a front fork, referred to as a “springer” fork, includes a spring suspension operating between two pairs of parallel fork members in a well known manner.
Other suspension systems may use air in place of or in addition to the spring. These systems are often called air-ride suspension systems, and may use rubber air bags as springs and may further be charged using a compressed air system. Compressed air may be varied to control the ride height and the ride characteristics (i.e. firmness) of the motorcycle. However, the ride will inherently get firmer as the air pressure increases in these systems.